Funnily enough, my father is at the 11th international symposium on “Ground Freezing” in London at this very moment. Most of the time you have to freeze the ground before you can drill. He has been working in specialist civil engineering for many years and builds tunnels all over the world. He has told me many exciting and hard-to-believe stories about tunnel construction and tunnel boring machines. E.g. during the w…
AFAIK it was not just the cost, but the problem of two TBMs (British and French) meeting head to head. The british were driven out of the way so that the French ones could proceed.